1. Field of the Invention
The present invention relates to an anesthetic agent analyzer for determining the amount of carbon dioxide, nitrous oxide and other anesthetic agents contained in the respiratory gas of an anesthetized patient. More particularly, the present invention relates to a multichannel mainstream anesthetic agent analyzer which measures the partial pressures of constituent gases in a respiratory gas stream without using any moving parts and displays the representative gas information on a display.
2. Brief Description of the Prior Art
It is frequently of critical importance to monitor the concentration of carbon dioxide (CO.sub.2) in the gases inspired and expired to/from a patient under anesthesia, for expired CO.sub.2 concentration is a reliable indicator of the carbon dioxide concentration in the arterial blood. In a clinical setting, monitoring expired CO.sub.2 prevents malfunctions in anesthesia rebreathing apparatus from going undetected and delivering excessive amounts of CO.sub.2 to the patient. Rebreathing of anesthetic gases is very cost effective and environmentally desirable, but accurate CO.sub.2 concentrations are difficult to maintain in the patient circuit without a concentration monitor.
Numerous CO.sub.2 concentration monitors have been described in the prior art which direct infrared radiation through a sample of a gaseous mixture and measure the incident radiation illuminating a detecting device, thereby obtaining a measure of the infrared absorption of the CO.sub.2 gas. Electrical signals produced by the detecting device are indicative of the infrared absorption of the CO.sub.2 gas and can be processed to produce an output indicating the concentration of the CO.sub.2 component in the gas being analyzed. This type of gas analyzer operates on the principles that CO.sub.2 or any other gas being measured exhibits substantially increased absorption characteristics at specific wavelengths in the infrared spectrum and that higher gas concentrations exhibit proportionally greater absorption.
Analysis of the inspired and expired respiratory gases of a patient also provides information regarding the amount of nitrous oxide (N.sub.2 O) and other anesthetic agents the respiratory gas contains. Anesthetic agents are typically present as a single agent gas or as a mixture of a number of agent gases during transition from one type of agent gas to another. Some of the most common anesthetic agent gases, other than nitrous oxide, are halothane, ethrane, isoflurane, desflurane and sivoflurane. Such agent gases when administered to a patient must be carefully controlled by the anesthesiologist because of the great risk of supplying too much or too little agent gas.
Previously, the gas analyzers that were used to measure agent gases were not the same devices that were used to measure end-tidal CO.sub.2 and N.sub.2 O. These devices have included mass spectrometers and non-dispersive infrared gas analyzers. Mass spectrometers that are used for such gas measurements are usually part of operating room suites in which one spectrometer is shared among many rooms to measure a multiplicity of gases. However, a mass spectrometer has the disadvantages of cost, maintenance and calibration requirements, slow response time, and non-continuous measurement. Infrared gas analyzers, on the other hand, have the ability to measure the concentrations of inspired and end-tidal CO.sub.2 and N.sub.2 O in real-time. Prior art nondispersive infrared gas analyzers have also included features for making CO.sub.2 and N.sub.2 O cross-channel detection as well as temperature and collision broadening corrections to their partial gas pressure measurements. Some of these corrections have been made automatically by the analyzers, while others have been made manually by the operator.
Such non-dispersive infrared gas analyzers generally have two configurations. The first type, and more common, is the sampling or side-stream type which diverts a portion of the patient's respiratory gas flow through a sample tube to the infrared analyzer. The second type, or mainstream type, mounts on the patient's airway and uses a portion of the airway as the sample chamber. A mainstream gas analyzer is often desirable since a sampling system with its pumps and filters and the like is not necessary, thereby substantially reducing the cost of the device. The present invention is directed to such a mainstream infrared gas analyzer, although those skilled in the art will appreciate that the techniques of the invention may also be used with a sidestream configuration such as that described in a commonly owned patent application to Braig, et al., U.S. patent application Ser. No. 07/976,145 filed Nov. 10, 1992.
In typical infrared gas analyzers, the wavelength band of the beam of infrared energy passing through a sample cell containing the unknown gas mixture or through a portion of the main airway is changed periodically by the interposition of one or more filters in the path of the light beam. Typically, this is accomplished by providing a rotating filter wheel containing a plurality of filters which each pass only that radiation in a narrow band corresponding to a characteristic absorption wavelength of a particular gas of interest. Another filter may also be used as a reference filter at a wavelength band close to but not substantially overlapping the characteristic absorption wavelength band of any of the gases present in the sample cell or in the expired air in the mainstream airway. Gas analyzers with such filter wheels are described by Passaro et al. in U.S. Pat. No. 4,692,621; by Conlon et al. in U.S. Pat. No. 4,914,719; by Williams in WO 90/04164; and by Flewelling et al. in U.S. Pat. No. 5,046,018. Such infrared gas analyzers also commonly use a chopper wheel for chopping at a predetermined frequency the infrared light passing through openings in the sample cell or mainstream airway and a source aperture aligned with the reference cell and gas pathway. The chopped light passes through the openings in the detector aperture aligned with a reference cell in the gas pathway to the remaining portions of the assembly so as to provide synchronization for subsequent processing. Such a multi-channel gas analyzer is described, for example, by Corenman et al. in U.S. Pat. No. 4,907,166.
Gas analyzers of the type described in the aforementioned patents usually continuously reference the radiation detected in the characteristic bands to radiation detected at reference levels (i.e., a non-absorbed wavelength with a dark or totally blocked level). By doing so, the effect of drift is minimized and the effect of background noise is reduced. As known to those skilled in the art, drift can occur as a result of contamination on the windows in the sample cell or in the windows of the mainstream airway adapter which will attenuate the radiation passing therethrough and which can be interpreted erroneously to indicate the presence of the gas to be detected in the gas sample. Drift can also be caused by shifts in the output of the detector and temperature changes in the source of the infrared radiation.
Another source of error for infrared gas analyzers is the presence of certain gases or combinations of gases in the measured airstream or sample cell which have absorption bands which substantially overlap. This is undesirable, for there are instances where the gases that need to be measured simultaneously have very significantly overlapping absorption bands in the infrared range. A well-known example is the case of the anesthetic halocarbons: halothane, ethrane (or enflurane), and isoflurane. These three gases, which are hydrocarbon derivatives, have relatively weak absorption bands bunched together in the 3.3-3.5 micron (middle infrared) range, much like other hydrocarbons, and thus have modulations in the middle infrared range which are hardly sufficient for a useful measurement. This leads to poor sensitivity, a bulky sample chamber, and vulnerability to interference from other gases. However, as will be noted in more detail below, these gases also have strong but overlapping absorption bands in the far infrared range (beyond the 3.3-3.5 micron range) extending all the way up to 15 microns. Since these anesthetic hydrocarbon derivatives have much stronger absorption in the 6-15 micron (far infrared) range, it is desired to develop a measuring instrument which can measure the absorption of infrared light in the 6-15 micron (far infrared) range, despite the overlapping characteristics of the absorption bands of the halocarbons at these wavelengths.
Accordingly, the present invention is intended to measure the concentrations of the anesthetic agents in the far infrared wavelength range so as to provide a more precise measurement of the concentrations of these anesthetic agents than has previously been possible. It is also desired to develop an anesthetic agent analyzer which, unlike the above-mentioned devices, has no moving parts and is relatively inexpensive. In particular, an anesthetic agent analyzer is desired which can measure the concentration of anesthetic agents in the far infrared wavelength range in a system which does not use rotating filter wheels or choppers as in the aforementioned prior art patents.
Infrared gas analyzers have previously been described which do not require a motor driven chopper wheel or rotating filter wheel for its operation. For example, Aldridge discloses in U.S. Pat. No. 4,772,790 an optical gas analyzer which uses thermopiles as optical detectors. The thermopiles are formed of an array of interconnected thin films deposited on a heat insulative substrate to form a multitude of thermocouples. The array is configured such that infrared light impinges upon a number of the thermocouples, while the remainder of the thermocouples are shielded from the infrared light and employed to compensate each thermopile output signal for changes in ambient temperature. Similarly, Junkert et al. disclose in U.S. Pat. No. 4,722,612 an infrared thermometer which shields one thermopile from the incident infrared radiation and uses its output to compensate for changes in ambient temperatures. Of course, both of these detectors require close matching between the detecting thermopiles and the "shielded" thermopiles in order to prevent mismatch errors.
Another infrared gas analyzer which does not require a motor driven chopper wheel or filter wheel for its operation is described by the present inventors in U.S. Pat. Nos. 5,081,998 and 5,095,913, which are also assigned to the present assignee. The present inventors therein describe an optically stabilized, shutterless infrared capnograph which provides the absolute concentrations of the constituents of the respiratory airstream of a patient without the thermal drift problems normally associated with thermopile detectors. The detector described in these patents eliminates the need for a mechanical shutter to modulate the incident infrared beam and the need for a modulated source by providing a unique arrangement whereby paired thermopiles are connected in series opposition to each other and are preceded by an analytical reference filter for passing a desired wavelength. A neutral density filter is also placed in the optical path of one of the thermopiles in the pair to attenuate the incident light, and the difference between the outputs of the thermopiles is used to eliminate the effects of background thermal noise so as to increase the reliability and response time of the device.
As illustrated in FIG. 1, such an optically stabilized infrared energy detector comprises a pair of thermopile detectors 100 and 102 which are connected in series opposed relation to each other. As shown, while infrared light impinges upon both of the infrared detectors 100 and 102, reference element 102 is disposed behind a neutral density filter (as shown by phantom outline) which attenuates a portion of the incident infrared radiation. As a result of this arrangement, the background temperature effects equally affect both detectors 100 and 102 and cancel when the outputs are summed. The sum output of operational amplifier 104 is thus unaffected by variations in ambient temperature and the like.
An exploded view of an optically stabilized infrared energy detector is shown in FIG. 2. As shown, the optically stabilized infrared energy detector 200 comprises an aperture 202 which is placed over analytical filters 204 including CO.sub.2 filter 206, N.sub.2 O filter 208 and a reference filter 210. A neutral density filter (not shown) is also placed over one channel of each analytical filter 204. Two substrate layers 212 preferably formed of antimony and bismuth are then placed on either side of a thin film thermopile layer 214 which includes thermopiles for detecting the incident infrared energy which has passed through the apertures 202, the analytical filters 204, the neutral density filter, and the openings in the substrates 212 so as to impinge upon the respective thermopiles in thermopile layer 214. A foil background 216 is also provided on the back of the lower substrate 212 so as to prevent the impingement of infrared energy from the back of the detector 200 onto the thermopiles of thermopile layer 214. A bead thermistor 218 is also provided behind foil 216 to measure the absolute substrate temperature for use in computing the ratio between the incident signals after cancellation of thermal effects. The entire assembly is then placed upon a header assembly 220 via header pins 222 as illustrated.
The optically stabilized infrared energy detector illustrated in FIG. 2 incorporates six coplanar detector channels, three of which serve as reference channels and three of which serve as analytical channels. As shown in FIG. 1, each reference detector in a reference channel is connected to its analytical partner in a series opposed configuration. It is desired to modify this configuration such that all six of the illustrated channels may be used as independent analytical channels, thereby permitting three or more additional anesthetic agents such as ethrane, halothane, and isoflurane to be detected with the same thermopile layer 214. However, it was discovered by the present inventors that the system of FIGS. 1 and 2 had to be substantially geometrically reconfigured into a new topography in order to permit six or more independent analytical channels to be used with a comparable thermopile assembly. As will be described in detail below, a new topography has accordingly been developed in accordance with the present invention.
Accordingly, a discriminating anesthetic agent analyzer is desired which has no moving parts and thus overcomes the problems of the aforementioned prior art agent analyzers and which is small, inexpensive and simple to operate. A discriminating anesthetic agent analyzer is also desired which is light in weight and durable so that it may be used in a mainstream configuration. An anesthetic agent analyzer is further desired which is accurate and which does not require excessive calibration. In addition, a discriminating anesthetic agent analyzer is desired which improves upon the topography of the above-mentioned optically stabilized infrared energy detector so that more analytical channels may be disposed in the same relative area. The invention described herein has been designed to meet these needs.